This invention relates to the preparation of 1,3-propanediol based polyesters such as polytrimethylene terephthalate. In one aspect, the invention relates to a process for reducing the tendency of polytrimethylene terephthalate pellets to stick together during storage or transportation, particularly at elevated temperatures such as encountered in warm climates. In a further aspect, the invention relates to the reduction of fines in the manufacture of polytrimethylene terephthalate.
Polytrimethylene terephthalate is a polyester useful in fiber applications in the carpet and textile industries. The manufacture of polytrimethylene terephthalate involves the condensation polymerization of 1,3-propanediol and terephthalic acid to a polymer having an intrinsic viscosity (IV) of about 0.5 to 1.0 dl/g. The polymer melt is discharged from the bottom of the melt reactor and extruded through an extrusion die into strands. The strands are quenched in cold water and cut into pellets for storage or transportation.
It has been found that polytrimethylene terephthalate pellets tend to stick together, or "block," during storage or shipping at temperatures above the polymer Tg (about 45.degree.C.), which can be reached during the summer in a silo or rail car. Agglomeration of the pellets can also occur during drying using a hopper-type dryer.
It is therefore an object of the invention to produce polytrimethylene terephthalate pellets which are resistant to blocking during storage or shipping. It is a further object to produce polytrimethylene terephthalate pellets which can be dried in a hopper without agglomeration. It is a further object of one aspect of the invention to reduce fines production in the manufacture of polytrimethylene terephthalate.